The present invention relates to an electronic photocopying and printing technique.
In an engraving apparatus for printing rastered or non-rastered images, an image to be reproduced is scanned by the photo-electric scanner point by point and line by line, and the image signals so obtained corrected according to the requirement of the photocopying process. The corrected image signals control a recording member, which generates the rastered or non-rastered print on a carrier. In an engraving apparatus the formation of the raster point on a cylinder is accomplished by means of an engraving stylus of an electromagnetic engraving means, or by the energy beam of a generator of such an energy beam, while in a scanner the reproducing means is a light modulated light source for rastered or non-rastered illumination of films or printing plates.
The images to be reproduced are, in most cases, combinations of continuous tone pictures having soft contours (small changes in tone value) or letters or characters, such as graphical representations having hard contours (large changes in tone values).
The exact reproduction of contours in a rastered print results in particular problems. While a contour may be arbitrarily positioned with respect to a scanner, such a contour can normally be reproduced by the raster points or pixels successively arranged in the recorded raster only approximately. For example a contour subtending a transverse angle with the direction of reproduction may be reproduced showing an annoying step structure.
In order to smooth contours of this type it is already known to displace those raster points which are positioned within the region of the contour from their respective exact positions in the printing raster. In the known solution the position of a contour on the scanning side is, however, determined only in a coarse manner, and on the reproducing side the raster points are displaced merely at right angles to the direction of scanning, so that the reproduction of any contour approximately perpendicular to the scanning direction cannot be corrected at all.
When using an energy beam to generate raster points, it is possible to displace raster points in an arbitrary direction from the printed raster, as an energy beam may be deflected free of any inertia effects, and can be rapidly switched in or out, so that contours, which subtend an arbitrary angle with the printing raster, can be smoothed by displacement of the raster points. It is a precondition for such a solution, however, that the position of the contour is known not only with respect of the exact passage of this contour through the point or region being scanned, but also with respect to the direction it subtends with the direction of scanning, so that this angle must be determined exactly, which has not yet been satisfactorily accomplished hitherto.